The third generation partnership project (3GPP) has developed standards for a cellular communication system, referred to as long term evolution (LTE). LTE uses a radio access network (RAN) that is different from the RAN used in GSM and UMTS networks. Mobile stations, which are usually termed user equipment (UE) can operate both with the GSM EDGE RAN (GERAN) and the UMTS Terrestrial RAN (UTRAN).
Users demand voice connections to be provided over the RAN. LTE, unlike GSM and UMTS, does not support Circuit Switched (CS) technology, which has conventionally been used for voice connections. It is expected that an IP Multimedia Subsystem (IMS) will be used for voice connections in LTE. Consequently, a UE will be able to make voice connections using CS technology over GSM or UMTS networks or use Packet Switched (PS) access, for example using Voice over Internet Protocol, VoIP, over LTE.
Whilst existing GSM systems have PS functionality, the GERAN does not support the necessary bandwidth and quality of service (QoS) features required for VoIP. A UMTS system can also provide packet switched services, but some systems may not provide the necessary bandwidth and the QoS features required for VoIP. For example, this may depend on whether specialist bandwidth control equipment is employed in the UTRAN.
An existing IMS cannot determine whether to deliver a mobile terminating call over PS access or via CS technology, without incurring some extra call setup delay. Additionally or alternatively, extra functionality may be required in the mobile switching centre (MSC) in the currently serving or visited network, for example an IMS centralised services functionality. Reducing this call setup delay is difficult without affecting other parts of the network.
One option is for the IMS to try delivering the voice connection over PS and, in the case of failure, revert to CS provision over GERAN or UTRAN if the UE can access these networks. However, this adds delay to mobile terminating call setup times. Moreover, such connections need to be made within a predefined time period and this additional delay could cause a time-out at the originating end, leading to the call being dropped at the worst case.
An alternative solution is for the UE to keep the core network continually informed with current VoIP capability. However, this leads to increased signalling, which is undesirable and GSM, UTRAN and LTE already include functionality to reduce signalling. For instance, base stations (equivalently Node B in UMTS or eNodeB in LTE) are grouped together in Routing Areas. The UE does not inform the core network of a UMTS system when it moves between access via GERAN and UTRAN, if these two RANs both share the same Routing Areas. Similarly, the UE does not inform the core network of an LTE system when it changes between LTE and GERAN or UTRAN if the UE and the currently serving or visited network have instructed the UE to use the “Idle mode Signally Reduction” (ISR) feature, which is defined in more detail in 3GPP TS 23.401 . Network operators find ISR a desirable technology, since its use improves network efficiency.
Providing the IMS with the information necessary to optimally route voice connections without a significant reduction in efficiency is a challenge.